Disk drive memory systems have been used in computers for many years for the storage of digital information. Information is recorded on concentric tracks of a magnetic disk medium, the actual information being stored in the form of magnetic transitions within the medium. The disks themselves are rotatably mounted on a spindle. Information is accessed by a read/write transducer located on a pivoting arm that moves radially over the surface of the rotating disk. The read/write head or transducer must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information.
During operation, the disks are rotated at very high speeds within an enclosed housing using an electric motor generally located inside a hub or below the disks. Such spindle motors may have a spindle mounted by two ball bearing systems to a motor shaft disposed in the center of the hub. The bearing systems are spaced apart, with one located near the top of the spindle and the other spaced a distance away. These bearings support the spindle or hub about the shaft and allow for stable rotational relative movement between the shaft and the spindle or hub while maintaining accurate alignment of the spindle and shaft. The bearings themselves are normally lubricated by highly refined grease or oil.
The conventional ball bearing system described above is prone to several shortcomings. First, vibration generated by the balls rolling on the bearing raceways is a problem. This is one of the conditions that generally guarantees physical contact between raceways and balls, in spite of the lubrication provided by the bearing oil or grease. Bearing balls running on the microscopically uneven and rough raceways transmit the vibration induced by the rough surface structure to the rotating disk. This vibration results in misalignment between the data tracks and the read/write transducer, limiting the data track density and the overall performance of the disk drive system. Further, mechanical bearings are not always scalable to smaller dimensions. This is a significant drawback, since the tendency in the disk drive industry has been to shrink the physical dimensions of the disk drive unit.
As an alternative to conventional ball bearing spindle systems, much effort has been focused on developing a fluid dynamic bearing (FDB). In these types of systems, lubricating fluid, either gas or liquid, functions as the actual bearing surface between a shaft and a sleeve or hub. Liquid lubricants comprising oil, more complex fluids, or other lubricants have been utilized in such fluid dynamic bearings.
The reason for the popularity of the use of such fluids is the elimination of the vibrations caused by mechanical contact in a ball bearing system and the ability to scale the fluid dynamic bearing to smaller and smaller sizes. In designs such as the single plate FDB, two thrust surfaces generally are used to maintain the axial position of the spindle/motor shaft assembly. Such a configuration maintains axial position; however, this configuration does not aid in reducing the power required by the FDB at start up.
To control axial position of the spindle/motor shaft assembly while lowering power consumption, one of the thrust surfaces may be removed from the FDB and replaced with a magnetic force to constrain the motor's axial movement. This typically involves adding a magnetic circuit to the assembly consisting of a magnet fixed to the hub, sleeve or base that attracts (or repels) the facing motor hub, sleeve or base. The magnetic circuit may be accomplished either by the addition of a magnet of by offsetting the stator magnet relative to the stator. However, while effective, this bias force may induce undesirable harmonics that produce acoustic noise in the motor.
Thus, there is a need in the art for a magnetically biased fluid dynamic bearing motor that minimizes acoustic noise emanating from the motor.